Field of the Invention
This invention relates to a process for injection compression molding ultra high molecular weight polyethylene of low melt fluidity.
Ultra high molecular weight polyethylene are so excellent in impact strength, abrasion resistance, self-lubricating property, chemical resistance and the like that they are widely used as engineering plastics. However, these resins have much higher melt viscosity and poorer fluidity than ordinary thermoplastic resins, and therefore, it is very difficult to shape these resins by conventional extrusion molding, injection molding or injection compression molding. As a result, these resins are usually shaped by compression molding or sintering molding, but the working efficiency is so poor that a shaping method having a short shaping cycle is desired and needed.
When ultra high molecular weight polyethylene is heated to an elevated temperature to reduce the melt viscosity and ordinary injection molding is effected, the molecular weight of the resin is lowered by thermal degradation, and therefore, the resulting shaped article is not practical because the inherent excellent properties are deteriorated.
On the other hand, there are so-called "injection compression molding methods" in which a resin is injected into a mold and then compressed. There may be mentioned the method in which, during injection of a plastic material into a cavity of a closed split mold, the cavity of said split mold is slightly expanded and after completion of the plastic material, the split mold is compressed to reduce the cavity of the mold (Japanese Patent Publication No. 1664/1965); a method in which a small gap is preliminarily formed between molds, a resin material is injected the gap is maintained until the injection is complete, and the material on the mold cavity surface is compressed and shaped simultaneously with completion of the injection (Japanese Patent Laid-Open No. 14657/1977), a method in which a cavity is filled by injecting at a high speed a molten thermoplastic resin having the same volume as a preliminarily determined cavity volume and then the resin of a volume of 0.5-3 times the cool shrinkage volume of said molten resin injected at a high speed is additionally injected at a low speed to open the mold followed by mold-closing accompanying cooling and solidifying of the molten resin in the mold (Japanese Patent Laid-Open No. 21258/1978) other methods are known.
These methods are all concerned with molding of ordinary thermoplastic resins. When these methods are employed for injection molding of thermoplastic resins of poor melt fluidity, in particular, to produce many pieces by one shot, articles molded according to the methods of the above mentioned Japanese Patent Publication No. 1664/1965 or Japanese Patent Laid-Open No. 21258/1978 suffer from layer-like exfoliation. Japanese Patent Laid-Open No. 14657/1977, also identified above is concerned with a method of the production of a large number of thin articles by one shot using ordinary thermoplastic resins. When this method is used for the purpose of the present invention, weights and shapes of the molded aritcles vary to a great extent resulting in production of poor articles.
In order to improve the molding methods, there was proposed a process for injection molding ultra high molecular weight polyethylene which comprises injecting the polyethylene into a mold cavity having a volume 1.5-3.0 times that of the injected polyethylene at a shear rate of at least 50,000 sec..sup.-1 measured at the gate of the injection nozzle and then compressing the mold cavity to a volume less than twice that of the injected polyethylene (Japanese Patent Laid-Open No. 81861/1976). This process enables ultra high molecular weight polyethylene to be injection-molded for the first time. This process gives molded articles of good quality and appearance when used for producing one piece by one shot, but gives molded articles which fluctuate in weight and shape when used for producing many pieces by one shot. In order to obtain molded articles free from such fluctuations using the above mentioned process, it is necessary to make the runner distance and volume the position of gate uniform. However, such molds are difficult to obtain by manufacture according to the design drawing. They can be obtained only by improving the runner, gate or the whole design of the mold by trial and error. Further, it is very difficult to produce large numbers of the same mold.